Although soap can be made by a number of different techniques, today its commercial manufacture basically involves either some type of batch (kettle) saponification or a continuous process which includes the splitting of fats into fatty acids and glycerine and then the neutralization of such fatty acids with caustic (usually either sodium hydroxide or potassium hydroxide) containing the proper amount of water to yield a neat soap containing about 30 percent by weight of moisture. While the most modern way to make soap is neutralizing fatty acids, considerable soap is still made by batch techniques which involve "cold process saponification", "semi-boiled saponification" and a so-called "kettle process". The cold process saponification is the simpliest of the batch procedures and since neither lyes nor nigre are separated, the glycerol and impurities from the fats remain in the soap. The charge of fat is simply melted in a vessel equipped with a mechanical stirrer and the calculated amounts of caustic soda solution is added with vigorous stirring. The fats and oils are mixed for a short time, usually from about ten minutes to one hour, or before the mix becomes too viscous to pour. At this time the saponification is about 90% complete. The mix is then poured into a frame and stored about two days to a week until hard. During this aging period, the saponification is completed. The semi-boiled saponification technique is similar to the cold process, although a higher temperature is used to speed saponification and permit adjustment of the alkali content before framing. The fat charge and alkali (which may be caustic potash when soft soaps are desired) are thoroughly mixed at 160.degree.-175.degree. F. until the soap becomes smooth. No glycerine is recovered in this process. The kettle process usually involves recovery of the glycerine. In this process fat and a relatively weak solution of sodium hydroxide are pumped into the kettle simultaneously. As soon as the dilute caustic mixes with the fat, saponification starts. The liquid mass is boiled by the admission of steam at the bottom of the kettle and as saponification proceeds, stronger caustic is added gradually until the saponification is almost complete. The soap is then "salted out" or "grained out" by the addition of a large amount of salt; the sodium soap, bein insoluble in the concentrated salt solution and of a lower density rises to the top of the kettle and the salt solution containing salt, glycerine, impurities and excess alkali collect at the bottom of the vessel. The bottom brine layer is drawn off and then water and an excess of lye are added to the soap remaining in the kettle. The mixture is boiled with steam to saponify the last traces of fat. The solution which collects at the bottom of the kettle during the subsequent settling process is drawn off. Brine is then added to the soap; the mass is boiled and allowed to stand until brine washed soap rises to the top of the kettle. The brine washing is repeated several times with fresh brine until the excess alkali and glycerine in soap are reduced to a minimum. The neat soap is then sent to dryers such as a Proctor-Swartz.
No matter what soap making procedure is employed, the end product is neat soap which is usually subjected to further processing. For example, when neat soap is to be further processed to form bars or flakes, the water content of the neat soap must be reduced to the range of about 10-20 percent by weight. This drying can be accomplished in a number of ways. In one procedure the neat soap flows onto a so-called chill roll which spreads the fluid soap into a thin film which then solidifies. The solidified soap film is removed in ribbon form and then oven dried to the required moisture content. More modern techniques utilize vacuum spray dryers to reduce the moisture content to a proper level. Following drying, the soap is passed to an amalgamator where perfume, color and other soap addivites are mixed into the soap mass and from there the soap can be milled and plodded. The final steps to bar soap manufacture include the extrusion of the plodded soap through a tapered outlet to form a continuous log followed by cutting, stamping and finally packaging. It will be appreciated that in conventional soap bar/soap flake manufacturing practices, the drying and plodding steps are energy-intensive and very time consuming and it would be extremely desirable to develop a soap-making process which would either substantially reduce or eliminate the drying operation which is currently required to produce most soap products.
Various proposals have been advanced to solve the problem of producing low moisture soap by eliminating or substantially reducing the energy required for drying but so far as we know none have really proved to be commercially feasible. For example, U.S. Pat. No. 2,730,539 discloses a method of saponifying fat such as tallow or vegetable oils with caustic to form a low moisture content soap using a "muller" type mixer. The soap making ingredient, such as tallow, and a solution of caustic soda are introduced into the muller mixer and subjected to a shearing and smearing action by the mulling action of the heavy wheels rolling over the materials in the pan. According to the patent, a high order of mechanical pressure is applied to the soap-forming ingredients which results in a soap which is said to be suitable for plodding and then stamping into bars.
Also, U.S. Pat. No. 3,657,146 discloses a process for the direct production of soap from fatty acids and caustic in a pressure vessel at about 2 to 10 atmospheres and at a temperature ranging from 120.degree. to 180.degree. C. The process is said to produce a soap having not more than about 25 percent water content and in this connection example 2 of said patent shows a soap which contains 9 percent of free fatty acids and 145 percent of water.
In addition, the use of a two-stage saponification procedure is disclosed in U.S. Pat. No. 2,753,363. The initial reaction takes place between the fatty acids and a dry, alkali metal carbonate such as sodium carbonate to achieve a partial saponification. Following this the partially saponified mass is treated with aqueous alkali metal hydroxide to complete the reaction.
U.S. Pat. No. 1,722,687 discloses the use of a high speed centrifugal pinned disc mill to make framed soaps, soft soaps and dry soap powders. In the process the soap making ingredients are introduced into the mill and the lower rotating disc is run at very high speed causing a beating action of the reactants by the lower rotating pins and upper stationary pins.